The impact of wave-modified wind stress in an ocean circulation model

Abstract

A wave-related momentum flux parametrization is introduced in the ocean circulation model HYCOM. The physical process driving this parametrization is the growing and dissipation of the surface wave field. Two numerical experiments are conducted for the South Atlantic: one with the new wave effect included and a control run (without any wave information) used as a reference for comparison. The forcing fields (including the wave-modified ones) are produced based on ECMWF ERA-20C reanalysis data. A temporal and spatial characterization of surface stress under wave effects shows, in the mean, a lesser momentum flux for the whole domain (stress difference of -7%). Positive departures (wave-modified surface stress larger than the classic approach) are found during intense storm events mainly in the Southern region of the domain (up to +59% of stress difference). The long-term ocean response to the stress difference shows a strong SST signature in two particular areas: the Brazil-Malvinas confluence (difference of -0.9°C) and the Benguela upwelling (up to +1.1°C difference). The differences in SST are consistent with both the physical processes being modified by the waves (e.g. the Benguela upwelling being weaker due to an increased local Ekman pumping / weaker surface stress) and with similar numerical studies. The wave-related physical processes responsible for positive events are illustrated during two storm events: one in the Southern part of the domain and a second near the Brazilian Southern coast. Strong positive surface current differences are registered with values up to 0.56 m/s, representing 43% of the classic current approach. The difference is found correlated to the wave-age and the wave slope.